1. Field of the Invention
The invention relates generally to a membrane separation systems, modules and methods and more specifically to single-pass tangential flow filtration operation for concentration and diafiltration of feed streams.
2. Description of the Related Art
Ultrafiltration (UF) and microfiltration (MF) membranes have become essential to the separation and purification in the manufacture of biomolecules. Biomolecular manufacturing, regardless of its scale, generally employs one or more steps using filtration. The attractiveness of these membrane separations rests on several features including, for example, high separation power, and simplicity, requiring only the application of pressure differentials between feed and permeate. This simple and reliable one-stage filtering of the sample into two fractions makes membrane separation a valuable approach to separation and purification.
In one class of membrane separations, the species of interest is that which is retained by the membrane, in which case the objective of the separation is typically to remove smaller contaminants, to concentrate the solution, or to affect a buffer exchange using diafiltration. In another class of membrane separations, the species of interest is that which permeates through the filter, and the objective is typically to remove larger contaminants. In MF, the retained species are generally particulates, organelles, bacteria or other microorganisms, while those that permeate are proteins, colloids, peptides, small molecules and ions. In UF the retained species are typically proteins and, in general, macromolecules, while those that permeate are peptides, ions and, in general, small molecules.
Permeation flux, also referred to as flux, is the flow of a solution through a filter. The ability to maintain a reasonably high flux is essential in the membrane separation filtration process. Low flux can result in long filtration times or require large filter assemblies, resulting in increased cost and large hold-up volumes retained in the modules and associated filter system equipment. The filtration process itself induces the creation of a highly concentrated layer of the retained species on the surface of the membrane, a phenomenon referred to as “concentration polarization,” which reduces the flux from initial membrane conditions. In the absence of counter measures, the accumulation of retained particles or solutes on the surface of the membrane results in decreased flux and if not corrected the filtering process ceases to function efficiently. One conventional approach to overcoming the effects of concentration polarization in the practice of microfiltration and ultrafiltration is to operate the separation process in tangential flow filtration (TFF) mode.
TFF filters, modules and systems include devices having flow channels formed by membranes through which the feed stream flows tangentially to the surface of the membrane. The tangential flow induces a sweeping action that removes the retained species and prevents accumulation, thereby maintaining a high and stable flux. Because higher tangential velocities produce higher fluxes, the conventional practice of TFF requires the use of high velocities in the flow channels, which in turn result in very high feed rates. These high feed rates result in low conversion, typically less than 10% and often less than 5%. Low conversion means that the bulk of the feed stream exits the module as retentate in a first pass without being materially concentrated in the retained solutes. Since many UF separations require high concentration factors, as high as 99%, the retentate is typically recirculated back to the inlet of that module for further processing. Systems with recirculation loops are complicated by the requirement of additional piping, storage, heat exchangers, valves, sensors and control instrumentation. Additionally, these systems are operated in batch mode resulting in undesirable effects, including subjecting the feed solution to processing conditions for a long time, often several hours.
A conventional recirculation TFF process including a recirculation loop is shown in the process and instrument (P&I) diagram of FIG. 1. A TFF module 1 having a feed port 9, a retentate port 12 and a permeate port 10 receives a feed stream 7 from a batch tank 22 through a recirculation pump 6. Conventional TFF processes use commercially available TFF modules with flow channels of constant cross-section independent of where along the length of the channel the cross-section is measured. A feed compartment 2 is pressurized by the combined action of a pump 6 and backpressure valve 15 downstream of the retentate port 12. Pressure sensors 8 and 13 monitor the feed and retentate pressures, respectively. A permeate compartment 3 typically at or close to atmospheric pressure produces a permeate stream 11 from the permeate port 10 for further downstream processing or storage. A retentate stream 14 returns to the batch tank 22 through a heat exchanger 16. The heat exchanger 16 is often necessary to cool-down the retentate stream 14, which can heat up as a result of the pressure energy dissipated through, the backpressure valve 15. Although the temperature increase across the backpressure valve 15 is typically only about 1° F., the cumulative effect of recirculation can gradually increase the temperature of the batch by about 10 to 30° F. in the absence of an effective heat exchanger. To control the temperature of the batch, a temperature sensor 5 can be used to send a control signal 17 to a temperature controller 18, which in turn can automatically operate a flow control valve 19. The valve 19 controls the flow of cooling water 20 through the heat exchanger 16. Spent cooling water 21 returns to a central water chilling system (not shown). To control this process the flow rate of the feed pump 6 can be set according to the module supplier's recommendations followed by throttling retentate valve 15 until the desired feed pressure is obtained. Typically, these two process components need to be repeatedly adjusted to account for the increased viscosity of the feed stream 7 as the feed stream concentration increases as the separation progresses.
These conventional TFF processes possessing recirculation loops typically utilize flow rates greater than 4 liters/min/m2, and more typically less than 20 liters/min/m2. These high flow rates are typically necessary to obtain practical fluxes and result in low single-pass conversion, f, typically between 5 and 10%. This in turn can require that the recirculation pump 6 be very large and the pipes carrying the feed stream 7 and the pipes carrying the retentate stream 14 have a flow capacity 10-20 times larger than those carrying the permeate stream 11. The need for a heat exchanger 16 and associated instrumentation, large recirculation pump 6, and large-capacity feed and retentate pipes makes conventional TFF systems with recirculation loops complex and costly. Additionally, the large capacity of the recirculation loop can result in a large system hold-up volume (i.e. a volume which remains in the system when processing is complete). The hold-up volume is a factor that typically leads to yield losses and that limits the maximum concentration factor achievable with such systems. Finally, because the process shown in FIG. 1 is inherently a batch process it takes several hours to process the volume in the feed stream 7 from the batch tank 22 before the desired output is ready for further processing. As a result, the solution being separated is exposed to the process for a long time, which can be a particularly undesirable feature for sensitive solutions. Furthermore, in these conventional processes the operating conditions are typically repeatedly adjusted as the process progresses to accommodate the changing volume of the batch and the increase in viscosity that can result from the increased concentration of the feed solution.
Several attempts have been made to improve conventional TFF module performance by modifying flow channel topology. U.S. Pat. No. 4,839,037, Bertelsen et al., discloses a spiral wound module with a tapered channel for the purpose of maintaining relatively constant velocities. U.S. Pat. No. 4,855,058, Holland et al., discloses maintaining flow velocities constant as material permeates, as applied to spiral wound membranes; using reverse osmosis, ultrafiltration and micro filtration membranes, and it describes the control of flow velocities by changing the channel height, the channel width and the channel length. U.S. Pat. No. 6,926,833, and related U.S. Pat. Nos. 5,256,294, 5,490,937, 6,054,051, 6,221,249, 6,387,270, 6,555,006, all issued to van Reis, and U.S. Pat. App. Pub. Nos. 2002/0108907 and 2003/0178367, van Reis, disclose improving the selectivity of ultrafiltration, including maintaining a constant TMP along the channel length by establishing a tangential flow of the fluid media over a second surface of the membrane and using converging channels having decreasing cross-sectional area. U.S. Pat. No. 6,312,591, Vassarotti, et al., discloses a filtration cell for carrying out a tangential flow filtration of a sample liquid feeding a flow of sample liquid tangentially over the membranes such that each channel is connected in parallel. Each channel includes in its longitudinal direction a number of subsequent channel sections separated by transitional zones and is constructed and arranged such that the main flow direction in subsequent sections changes abruptly in the transitional zones. The cell operates using a conventional TFF process and recirculates the sample liquid through a loop.
In conventional membrane processes such as reverse osmosis, gas permeation and ultrafiltration the desired separation can be enhanced or its costs reduced by staging. Systems with permeate staging devices process the permeate from a membrane module or a number of modules into another membrane stage or module, as the feed stream of this second stage. This is generally done to further remove impurities from the permeate. In some gas separation processes both the permeate and the retentate from a module or series of modules is further processed in one or more membrane stages. Typical of these processes is that described in U.S. Pat. No. 5,383,957, Barbe, et al., which discloses producing pure nitrogen from air.
Retentate staging is used in both ultrafiltration and reverse osmosis processes practiced on a large scale. In retentate staging the retentate from a stage serves as a feed to the next stage. Typical reverse osmosis processes include two to four retentate stages. The transition between stages is typically defined by a change in total flow cross-section of the flow channels in each stage. Typically each stage contains three to six spiral wound modules in a vessel. In reverse osmosis it is usual for the overall process to permeate between 50% and 75% of the feed. In order to maintain a fairly uniform flow rate through the membrane modules, the number of modules in each stage is reduced to compensate for the reduced feed rate to that stage. This configuration is known in the art as a “Christmas tree” design. In ultrafiltration systems, feed-and-bleed stages are used. In this design the effluent from each stage is partly recirculated to the feed of the stage by means of a recirculation pump. These conventional reverse osmosis multi-stage systems generally do not exceed four stages because of the requirement for additional external piping, instrumentation, controls and pumps. The efforts to apply retentate staging to UF systems based on the use feed-and-bleed systems have been generally unsuccessful for bio-processing applications. These retentate staged systems use circulation pumps in each stage, and while they are used extensively in the food industry for very large scale processes, these systems are too complex for use in the pharmaceutical industry and too difficult to validate for compliance with regulatory requirements. Retentate staged systems are used for water purification by reverse osmosis. Almost universally these systems use spiral wound modules. Because of the low permeability of RO membranes, thick flow channels are used without reducing fluxes materially. Because the fluxes are low very long stages are needed to achieve a reasonable concentration factor per stage.
Attempts have been made to develop single-pass TFF processes, however these attempts often require very high pressures, a multiplicity of very long modules in series and with the use of circulation pumps or re-pressurization pumps between stages. The modules usually have channels with relatively large channel dimensions. These conventional attempts result in large hold-up volumes and the additional complexity of intermediate pumps, tanks, valves, and instrumentation. In addition to concentration, staged filtration processes are used to carry out diafiltration. The most common form of diafiltration in continuous processes is the “parallel” diafiltration in which diafiltrate is added to each stage. Counter current diafiltration is sometimes used to reduce diafiltrate requirements. In counter current diafiltration fresh diafiltrate is added to the last stage and permeate of each stage serves as diafiltrate to the preceding stage. In both these forms of staged diafiltration process the total amount of diafiltrate required to achieve a given degree of permeating impurity removal decreases as the total membrane area is subdivided into a larger number of stages. The limit on the number of stages used is the increased cost as the total membrane area is divided into smaller stages. Conventional diafiltration systems generally use recirculation pumps with diafiltrate injected just before the pumps and do not operate as a single-pass TFF process.
Thus, the need still exists for a simple tangential flow process suited for the needs of pharmaceutical and biotech processes which is able to yield high reliable flux and high conversion without the need of recirculation loops and intermediate pumps, and that can be readily driven by the low-pressure differentials between the feed and the permeate. It would be desirable to operate a bio-processing separation in a single pass mode without a recirculation loop while providing a high conversion with a relatively low hold up volume. It would be further desirable to operate the separation without the requirement of a high capacity feed pump and associated system interconnections. Operation of a diafiltration process in a single pass mode would also be desirable. It would also be desirable to reduce bio-processing system cost by reducing some of the system complexity by using more versatile separation modules.